1. Field of the Invention
The present invention relates to an inkjet printer having a borderless printing function, and more particularly to a paper edge sensing apparatus and a method of borderless printing which can improve a degree of precision in sensing paper edges such as a top edge, a bottom edge, a left edge, a right edge, and so on, in a borderless printing mode to reduce printing margins for the printer head to jet ink, to thereby reduce ink pollution and consumption due to unnecessary ink jetting.
2. Description of the Related Art
In general, an inkjet printer 10 is, as shown in FIG. 1, provided with a paper supply tray or cassette 1 in which sheets of paper are loaded, a pickup roller 2 supplying, sheet by sheet, the sheets of paper P loaded in the paper supply cassette 1, a convey roller 3 and pressure roller 3a conveying a sheet picked up by the pickup roller 2, a sheet detector 7 disposed between the pickup roller 2 and the convey roller 3 and provided with a sensor actuator 9 and an optical sensor 8, a printer head 4 mounted to a carrier 14 to form an image on a sheet conveyed by the convey roller 3, and a paper discharge roller 5 and a star wheel 5a externally discharging a sheet on which an image is formed. The respective rollers 2, 3, and 5 are driven by a driving motor (not shown) through a gear train.
In the operations of the inkjet printer 10 structured as above, electric power is first applied to the driving motor to start the printer 10. Accordingly, a driving gear 6 rotates the pickup roller 2, convey roller 3, and paper discharge roller 5. At this time, the pickup roller 2 rotates in a direction to pick up a sheet of paper by a dynamic power switching gear unit (not shown), and the convey roller 3 and the paper discharge roller 5 rotate in a direction opposite to a direction in which the sheet is conveyed.
The pickup roller 2 cooperates with a friction buckler (not shown) supplying the sheets of paper P one by one to pick up and convey one of the sheets of paper P loaded in the paper supply cassette 1 by a certain distance toward the convey roller 3.
When the sensor actuator 9 of the sheet detector 7 moves by the upper end, that is, the leading end of a sheet of paper so that the optical sensor 8 generates an ‘on’ signal, the driving motor is driven to rotate the driving gear 6 in a reverse direction. As the driving gear 6 rotates in reverse, the dynamic power switching gear unit releases from the pickup roller 2 to stop the driving of the pickup roller 2, and, at the same time, rotates the convey roller 3 and the paper discharge roller 5 in a direction the sheet is conveyed so that the sheet enters a base frame 12.
When the inkjet printer 10 prints in a general print mode placing margins on the sheet, a controller (not shown) counts a period of time during which the optical sensor 8 generates an “on” signal. When a predetermined period of time lapses, that is, when the leading end of the sheet passes down an ink jet position below ink jet nozzles 4a and enters up to a predetermined print position, the controller generates a print command to jet ink through the ink jet nozzles 4a of the printer head 4.
When the ink jet printer 10 prints in a borderless print mode placing no margin on the sheet, the controller generates a print command when the leading end of the sheet arrives at a distance having a predetermined top margin T upstream in a sheet convey direction A from the ink jet position of the ink jet nozzles 4a as shown in a solid line of FIG. 2.
As above, when the print command is generated, the printer head 4 slides to the left and right by means of the carrier 14 supported by a carrier shaft 15 so as to jet ink on the sheet or the base frame 12, to thereby form images.
Thereafter, when the lower end, that is, the rear end of the sheet passes through the sensor actuator 9 and the optical sensor 8 generates an “off” signal, the controller generates a print end command to place a predetermined bottom margin in a general print mode before the rear end of the sheet reaches the ink jet position of the ink jet nozzles 4a, but, in the borderless print mode, the controller generates the print end command after the rear end of the sheet is further printed, as shown in dotted lines of FIG. 2, as much as a distance having a predetermined bottom margin B from the ink jet position of the ink jet nozzles 4a. 
As above, the image-printed sheet is externally discharged from the printer by the paper discharge roller 5 and the star wheel 5a. 
A conventional inkjet printer 10 estimates the distance between the ink jet position of the ink jet nozzles 4a and the leading and rear ends of the sheet, based on the operation timing of the sheet detector 7. In other words, since the distance between the position of an end of the paper when it is detected and the position of the ink jets are known, and since the speed of movement of the paper is also known, the location of the paper can be estimated.
However, the ink jet nozzles 4a and the sheet detector 7, which are supported by different frameworks, may cause many position detection errors due to manufacture or assembly errors, that is, of a difference between an estimated position of the paper based on a predetermined time period between when the sheet detector operates and the controller instructs a print command as compared to the actual position of the paper.
Such position detection errors are usually not a problem in the general print mode placing margins on a sheet, but lots of problems can occur in the borderless print mode when no margin is placed on the sheet since the accuracy of determining the position of the paper must be estimated within a narrower range.
That is, in the borderless print mode, the print area 20 is set, as shown in FIG. 3, to have predetermined print margins, that is, top, bottom, left, and right print margins T, B, L, and R outside boundaries of top, bottom, left, and right edges 15, 18, 16, and 17 of a sheet, if print tolerance errors occur.
Accordingly, in the borderless print mode, ink jetted on the top, bottom, left, and right print margins T, B, L, and R of the print area 20 by the ink jet nozzles 4a is jetted on a base frame 12, causing a problem of contaminating the bottom surface of a sheet passing over the base frame 12.
Further, such print margins are generally set in a range of 2˜3 mm so that ink is unnecessarily jetted on the print margin area which increases a consumed amount of ink, to thereby increase the printer maintenance cost.